The invention concerns an optical emitter array with collimation optics in which a number of extended emitters are arranged side-by-side in the x-direction, with a specified divergence xcex1x in this direction and a center-to-center separation (Px) greater than the emitter size Ex and in which the collimation optics include a cylindrical lens array with a number of convergent cylindrical-lens surfaces each assigned to an emitter and having its cylindrical axis lying in the y-direction, arranged in front of the emitter array.
Such emitter arrays, with a number of optical sources that are linearly extended in the x-direction of width (Ex) running along the x-axis in the shape of a linear matrix with center-to-center distance (Px) appear, for example, in high-power diode laser bars. The combined width of these laser bars lies in the range of a few millimeters. For concrete applications, it is generally necessary to subject these discontinuous beam profiles formed from separate linear sections to a beam shaping such as a homogenization or a geometric cross-section transformation. The first step in this beam preparation is generally collimation, the goal of which is to achieve the greatest possible degree of divergence reduction over the entire active beam cross-section.
The collimation must take into account the strongly anisotropic divergence distribution of semiconductor lasers. Specifically, the divergence angle xcex1y in the y-direction (xe2x80x9cfast-axisxe2x80x9d) is relatively large while the divergence angle xcex1x in the x-direction (xe2x80x9cslow-axisxe2x80x9d) is comparatively small. This situation is handled by using cylindrical lenses to produce parallel rays in the different coordinates. For a laser bar with a number of emission centers, the fast-axis collimation is carried out by a single cylindrical lens lying in the x-direction and, because of the large divergence, positioned as closely as possible in front of the emitter array. For collimation along the slow-axis, a cylindrical-lens array is employed in which a cylindrical lens, whose width corresponds to the center-to-center distance (Px) of the emitter, is placed in front of each respective emitter in the beam direction.
A good collimation in the x-direction is fundamentally possible when the focal length (F) of the cylindrical-lens array is as large as possible relative to the emitter width (Ex). This, however, leads to a large overall length, which is particularly disadvantageous for micro-optic components and is therefore undesirable. This procedure has the additional disadvantage that ray bundles emerging from the emitters overlap one another in the x-direction at a distance (a) in front of the emitter surfaces and, as a result, a separate divergence reduction for individual emitters is no longer possible.
For a separate collimation of the individual emitters, a cylinder-lens array can of course be positioned within the overlap distance, a. From fundamental considerations, a divergence reduction under the assumption of smaller divergence in the slow-axis is only possible when the ratio of emitter size to center-to-center distance, Ex/Px less than 0.5, holds. For high-power diode laser bars, however, generally Ex/Pxxe2x89xa70.5 so that a separate collimation with a single cylindrical-lens array is fundamentally impossible.
An additional problem is that the beam cross-section still has a discontinuous energy distribution and so the high brightness of the emitter array is of only limited utility due to the great inhomogeneity.
Based on the problems illustrated above, the task of the invention is to provide an optical emitter array, in particular some laser bars, with collimation optics, which delivers a better beam quality, especially with regard to parallelism and homogeneity, while having a smaller overall length.
For solving this task, the invention proposes that a first cylindrical-lens array with focal length (F1) be positioned within the overlap distance (a) in front of the emitters at which the individual ray bundles emerging from the emitters overlap, in which the focal length (F1) is smaller then the overlap distance (a) and that a second cylindrical-lens array with focal length (F2) be positioned in a telescopic arrangement at a distance (b=F1+F2) in front of the first spherical-lens array.
According to the invention, the first and second cylindrical-lens arrays form an optical telescope whose eyepiece side is directed toward the emitter. Since the first cylindrical-lens array located there is maximally at the overlap distance (a) the overall result is a very short total length.
Since the front end of the collimation optics is located within the overlap distance (a), a separate collimation of each individual emitter is possible. The particular advantage of the invention consists in that an effective divergence reduction results nonetheless, even when Ex/Pxxe2x89xa70.5.
This advantage of the collimator-telescope arrangement according to the invention has to do with the fact that the first cylindrical-lens array produces a virtual magnification in an image plane of the emitter with width (Ex) to the width of the center-to-center distance (Px). This becomes the effective source size for the second cylindrical-lens array. Because of the constant beam-parameter product according to the Lagrange invariant (aperture x sine (divergence)=constant), the result is a divergence reduction by a factor of the magnification, M, of the telescopic arrangement. The approximation is valid for small angles, which is, however, no problem for the small divergence along the slow-axis.
A further essential advantage of the collimation optics according to the invention consists in that, by magnifying the sources to the center-to-center distance (Px), a continuously linear beam cross-section is produced in the x-direction. For a given emitter array, a high brightness is thereby produced. The homogeneous intensity distribution is particularly advantageous as an input beam for subsequent beam transformation devices.
For this embodiment of the invention, the overlap distance (a) observes the relation:   a  ≤                    (                              P            x                    -                      E            x                          )                    2        ⁢                  xe2x80x83                ⁢                  tan          ⁡                      (                          α              x                        )                                .  
In this formula, xcex1x is the divergence in the x-direction. For the focal length (F1) of the first cylindrical-tens array, the following value is set:       F    1    =            a                                    E            r                                p            r                          +        1              .  
Based on this value, the focal length (F2) of the second cylindrical-lens array is calculated as:       F    2    =            F      1        ⁢          xe2x80x83        ⁢                            P          r                          E          r                    .      
With the formulas described above, a cylindrical-lens collimator with two successive cylindrical-lens arrays that together form an imaging telescope is described for the first time.
The collimation efficiencies of the embodiment of the invention with an imaging-telescope arrangement mentioned above, can alternatively be realized by a telescopic arrangement with Fourier transform and field lens in which a first cylindrical-lens array (A) is positioned in front of the emitter (E) at a distance (z1) that is smaller then the overlap distance (a) at which the individual beams emerging from the emitter (E) overlap and a second cylindrical-lens array (B) is positioned at a distance (T) in front of the cylindrical-lens array (A) whereby the arrays (A and B respectively) together form a biconvex lens with resultant focal length (Fr=FA) and principal-plane distance (T=FA).
For the collimation, the same requirement holds with respect to the distance to the first cylindrical-lens array (A) as before with the imaging-telescope arrangement, namely, that ray bundles emerging from neighboring emitters (E) must not overlap.
In contrast to the first variant of the invention, however, no image results from the first cylindrical-lens array (A) but rather a Fourier transformation. The second cylindrical-lens array (B) has, in this arrangement, the function of a field lens.
This embodiment starts with the assumption that the maximal focal length (FA) can only be theoretically reached when Ex=0. In fact, however, the emitters (E) should be considered as extended sources that must obey 0 less than z1 less than FA in order to fulfill the requirement that beams emerging from neighboring emitters do not overlap in front of the collimation optics. According to the present embodiment of the invention, this is achieved in that the collimation as a whole results from a biconvex lens with the resultant focal length (Fr) whose principal-plane distance is T=FA/n (with n=1 for air). In this configuration, the second lens (B) serves as field lens.
From the correlation for two thin lenses with focal lengths (FA and FB) the resultant focal length (Fr) of the collimation system observes the relation:             F      r        =                  (                              F            A                    ⁢                      F            B                          )                              F          A                +                  F          B                -                  T          n                      ,
for T/n=FA, the result is: Fr=FA.
In addition, for T/n=FA:       z    1    =                              F          A                ⁡                  [                      1            -                                          F                A                                            F                B                                              ]                    ⁢              xe2x80x83            ⁢      or      ⁢              xe2x80x83            ⁢              1                  F          B                      =                  1                  F          A                    -                                    z            1                                F            A            2                          .            
With the resultant focal length (Fr) one then has a flexibly adjustable distance (z1) between the emitter (E) and the first cylindrical-lens array (A). The focal length (FB) of the second array (B), which serves as a field lens, compensates for this in accordance with the preceding relation.
In the just-mentioned biconvex lens arrangement, the first lens surface, which is formed by the first cylindrical-lens array (A) is responsible for the Fourier transformation of the ray bundle. The second lens surface, which is formed by the second cylindrical-lens array (B), contributes, as a field lens, to a narrowing of the emission angle.
Under certain conditions, when the individual emitters (E) of the emitter arrays are very small in the x-direction, it is useful, in regards to delivering the best brightness possible, to first reduce the divergence along the fast-axis, i.e. in the y-direction, and afterwards to collimate the slow-axis. This can be achieved according to the invention by locating between the emitters (E) and the cylindrical-lens array (A) a cylindrical-lens surface (C) whose cylindrical axis lies in the x-direction, i.e. parallel to the longitudinal axis of the emitter (E). In this way, since the first cylindrical-lens array (A) and the third cylindrical-lens surface (C) are combined into a monolithic component, the result is an advantageously small number of lens interfaces.
In order to eliminate residual divergence in the y-direction, i.e. along the fast-axis, a fourth cylindrical-lens surface, whose cylindrical axis lies in the x-direction, can be positioned between the first cylindrical-lens array (A) and the second cylindrical-lens array (B). This produces the same advantages as those already described above for the cylindrical-lens array (A) and the cylindrical lens (C). It is likewise useful to combine the second cylindrical-lens array (B) and the fourth cylindrical-lens surface (D) into a monolithic component.
Starting from the same fundamental problem, the invention presents an alternative embodiment, which likewise has two successive cylindrical-lens arrays for slow-axis collimation but which are, in contrast, optically divergent. In particular, it is envisioned here that a first cylindrical-lens array with focal length (FA) is positioned in front of the emitters at a distance (c) that is small relative to FA and a second cylindrical-lens array with the same focal length (FA) at a distance of this focal length (FA) from the first cylindrical-lens array.
In this embodiment of the invention, it is a matter of a Fourier arrangement in which no imaging results as with conventional telescopes. Here, the first cylindrical-lens array generates, in an intermediate plane, a source for the second cylindrical-lens array whose size, depending on the Fourier transformation, is dependent on the divergence of the original source, that is, the emitters of the laser bars.
This arrangement, which one can call a cylindrical-lens Fourier telescope, is just as effective, with respect to collimation, as the first-mentioned classic telescope arrangement. In addition, particular advantages arise for small emitter sizes (Ex) and short distances (Px). For these cases, the overlap distance (a) is sometimes so small that the overlapping in the slow-axis already occurs in the, generally intermediate, collimation cylindrical lens for the fast-axis. Fundamentally, this problem can be solved by making the fast-axis collimation lens smaller. By shrinking the aperture as well as the focal length, the divergence of the collimated ray bundles would, however, increase as a result in the direction of the fast-axis (y-direction). Here, the Fourier telescope arrangement offers the advantageous possibility, by shortening the distance (c), of moving the first slow-axis cylindrical-lens array according to the invention very close, theoretically down to 0 distance, to the emitters without having to accept a worsening of the collimation effect. This produces, in the first place, an extremely compact design. In addition, the possibility still remains, with an appropriate positioning of a y-cylindrical lens, to maintain good collimation in this direction.
In the cylindrical-lens Fourier telescope, one applies for the focal length (FA):       F    A    =                    P        x                    2        ⁢                  xe2x80x83                ⁢        tan        ⁢                  xe2x80x83                ⁢                  α          x                      .  
The cylindrical-lens arrays are preferably of one-piece construction. This holds as much for the telescope as well as for the Fourier arrangement. In particular, the first and second cylindrical-lens arrays can each be of one-piece construction. In addition, one can imagine bringing the optically active cylindrical-lens surfaces of the first and second cylindrical-lens arrays together into one monolithic optical component.
A preferred embodiment of the invention allows for arranging a cylindrical lens, whose cylindrical axis lies in the y-direction, in front of the emitter array as fast-axis collimator. This fast-axis collimator can, at the same time, have the form of an array and, as the case may be, once again be constructed in one piece with the first and/or second cylindrical-lens array. In this context, an advantageous embodiment envisions, for example, that a first monolithic optical element has the cylindrical-lens surface of the first cylindrical-lens array on its side facing the emitters for x-collimation and, on its other side, has a cylindrical-lens surface directed in the y-direction for collimation in that direction. A second x-cylindrical-lens array can then be positioned at the focal-length distance (FA). The result is an extremely compact design for x-y-collimation optics in which the x-divergence reduction results from a Fourier-telescope arrangement according to the invention.
Preferably, the cylindrical lenses have an aspherical form. The imaging quality in the intermediate plane is improved in that the resulting divergence is significantly reduced. As a result, the width of the ray bundle does not exceed the maximal aperture (Px).
With the devices according to the invention, the fundamentally new collimation method according to the invention can be adapted for application to emitter arrays, preferably high-power diode laser bars, for which Ex/Px is frequently greater than or equal to 0.5. This method envisions that, by means of a first cylindrical-lens array, the ray bundle of width (Ex) for each individual emitter is broadened to the center-to-center distance (Px) whereby a virtual source of width (Px) is generated in the intermediate plane of the second cylindrical-lens array. The particular advantage of this method according to the invention consists in that a separate collimation is carried out for each individual emitter. For this purpose, collimation arrangements, such as those presented as examples above, are inserted in front of the emitters within the overlapping distance at which the ray bundles begin to overlap because of their divergence. An alternative for this is that the first and second cylindrical-lens arrays are arranged in telescope configuration, that is to say, at a distance (b=F1+F2) which is the sum of their focal lengths, or in Fourier configuration whereby both cylindrical-lens arrays with common focal length (FA) are located at the focal-length distance (FA).
The method according to the invention overcomes the limitations that arise in principle from the application of separate cylindrical-lens arrays for each divergence direction, common until now.